Following recent acceleration and automation in cutting, importance has been given to the machinability of a steel employed for machine structural parts and a demand for so-called free cutting steels having improved machinability has risen. Further, the request for the strength of a steel material is becoming stricter. If the strength of a steel material is increased, the machinability thereof is deteriorated. That is, improvements in contradicting properties, i.e., high strength and machinability, are required for recent structure steels.
At present, steel materials which contain Pb, S and Ca, respectively, are known as ordinary-used free cutting steels. Among these steels, the Pb-containing free cutting steel which contains Pb exhibits excellent properties that it is lower in the deterioration of mechanical properties than a standard steel, it has improved chip disposability (the property capable of discharging chips more smoothly) in ordinary turning, and it is capable of lengthening the life of a tools employed for drilling, tapping, reaming, boring or the like. Furthermore, the Pb-containing free cutting steel facilitates discharging chips at the time of deep drilling to give (hole depth/drill diameter) ≧3 and is excellent in the prevention of the breakage of the tool due to sudden chip clogging.
In addition, various types of Pb composite free cutting steels are under development, which have the above excellent properties by adding elements such as S and Ca other than Pb.
However, the conventional Pb-containing free cutting steels has the following disadvantages.
Namely, although Pb is a quite effective element for the improvement of machinability of steels, it is an environmentally hazardous material. Due to this, because of a recent increase in interest in the environmental issues, it is desired to develop a steel material without Pb and comparable to the Pb-containing free cutting steel.
On the other hand, although there are conventionally known other free cutting steels without Pb, they cannot be replaced with the Pb-containing free cutting steel. It's because these steels have the following disadvantages.
For example, an S-containing free cutting steel which contains S has an improvement effect of lengthening the life of a tool for a relatively wide range of cutting; however, it is inferior to the Pb-containing free cutting steel in chip disposability. In addition, if a steel contains S, MnS which exists as an inclusion is extended during hot rolling or hot forging. Due to this, such a steel has a disadvantage in strength anisotropy, i.e. the mechanical properties of such a steel including impact strength are deteriorated as the direction is closer from an rolling direction to a right angle direction. Accordingly, it is necessary to suppress the S content of a steel material intended to be employed as a component which is considered to be given much importance to impact strength, which in turn provides insufficient machinability.
Further, a Ca-deoxidized free cutting steel in which the melting point of an oxide-based inclusion in the steel is lowered by Ca deoxidization, hardly influences the strength property of the steel material and exhibits an excellent effect of lengthening the life of a carbide tool in a high velocity cutting region. However, the Ca-deoxidized free cutting steel has little effect in machinability improvement other than the effect of lengthening the life of the carbide tool. Normally, therefore, the Ca-deoxidized free cutting steel is employed in combination with S or Pb so as to obtain all-round machinability.
There is a steel material described in Japanese Examined Patent Publication No. 5-15777 which illustrates an example in which the disadvantage of the S-containing free cutting steel, i.e. strength an isotropy, is improved by adding Ca and uniformly dispersing and distributing inclusions in the steel and, at the same time, the machinability of the steel is improved, opposed to the conventional Ca-deoxidized free cutting steels. In this case, the steel material is free from the disadvantage like the Ca-deoxidized free cutting steel has; however, it is required to add a large quantity of S to the steel material so as to ensure adequate machinability. In the above case, a sufficient quantity of Ca should be added to the steel material to control the form of the sulfide. However, in this case, Ca yield is lowered, which make it quite difficult to realize the quantity-production of steels.
Additionally, there is known steel materials described in Japanese Examined Patent Publication No. 52-7405 as an example of steels intended to attain the same effect as that of adding Ca described above. These are free cutting steels which contain one or two of Group I elements of Mg and Ba and one or more of Group II elements of S, Se and Te. Since O is actively added to these steel materials in a range of 0.004 to 0.012%, they might be low in fatigue strength. Besides, oxides in the steels increase by the active addition of O, thereby possibly deteriorating machinability such as drilling machinability.
Moreover, Japanese Examined Patent Publication No. 51-4934 discloses a free cutting steel which contains one or two of Group I elements of Mg and Ba and one or more of Group II elements of S, Se and Te, as well as a free cutting steel which selectively contains Ca. However, O is actively added to these steels in a range of 0.002 to 0.01%. Therefore, they might be low in fatigue strength. Besides, oxides in the steels increase by the active addition of O, thereby possibly deteriorating machinability such as drilling machinability.
Japanese Patent Publication No. 51-63312 discloses a free cutting steel which contains S, Mg and one or more elements of Ca, Ba, Sr, Se and Te. However, 51-63312 fails to concretely show the composition of the steel and insufficiently discloses the technique. In addition, since this steel is based on the assumption of Al deoxidization, there is fear that an Al content thereof exceeds 0.02%, no restriction is given to an O content thereof and fatigue strength is lowered. There is also fear that the quantity of oxides in the steel increase by the active addition of O, and the machinability such as drilling machinability is, therefore, deteriorated.
The present invention has been achieved in view of the above-stated conventional disadvantages and has an object to provide a lead-free steel for machine structural use, which does not contain Pb and is equal to or higher than the conventional Pb-containing free cutting steels in properties, excellent in machinability and low in strength anisotropy.